If we have a custom shape that derives from RadDiagramShapeBase and we rotate it at some degree (90) for example, and then resize it the diagram's selection adorner is misplaced with the shape.

Workaround:
Set RenderTransofrmOrigin = new Point(0.5, 0.5) to the shape.
Other workaround:
Derive the custom shape from RadDiagramShape, instead of RadDiagramShapeBase.